41 research outputs found

    Waveform Design for 4D-Imaging mmWave PMCW MIMO Radars with Spectrum Compatibility

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    4D-imaging mmWave radars offer high angular resolution in both azimuth and elevation, but achieving this requires a large antenna aperture size and a significant number of transmit and/or receive channels. This presents a challenge for designing transmit waveforms that are both orthogonal and separable on the receive side, as well as have low auto-correlation sidelobes. This paper focuses on designing an orthogonal set of sequences for 4D-imaging radar sensors based on PMCW technology. We propose an iterative optimization framework based on Coordinate Descent, which considers the Regions Of Interest (ROI) and optimizes a phase-modulated constant modulus waveform set based on weighted integrated sidelobe level on the required ROI and spectrum shaping. The optimization also accounts for the radar working adjacent to communication systems and other radar sensors. Simulation results are provided to demonstrate the effectiveness of the proposed method, which achieves low sidelobe levels and is compatible with spectrum constraints

    Coherent FDA Receiver and Joint Range-Space-Time Processing

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    When a target is masked by mainlobe clutter with the same Doppler frequency, it is difficult for conventional airborne radars to determine whether a target is present in a given observation using regular space-time adaptive processing techniques. Different from phased-array and multiple-input multiple-output (MIMO) arrays, frequency diverse arrays (FDAs) employ frequency offsets across the array elements, delivering additional range-controllable degrees of freedom, potentially enabling suppression for this kind of clutter. However, the reception of coherent FDA systems employing small frequency offsets and achieving high transmit gain can be further improved. To this end, this work proposes an coherent airborne FDA radar receiver that explores the orthogonality of echo signals in the Doppler domain, allowing a joint space-time processing module to be deployed to separate the aliased returns. The resulting range-space-time adaptive processing allows for a preferable detection performance for coherent airborne FDA radars as compared to current alternative techniques.Comment: 11 pages, 9 figure

    In-band-full-duplex integrated access and backhaul enabled next generation wireless networks

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    In sixth generation (6G) wireless networks, the severe traffic congestion in the microwave frequencies motivates the exploration of the large available bandwidth in the millimetre-wave (mmWave) frequencies to achieve higher network capacity and data rate. Since large-scale antenna arrays and dense base station deployment are required, the hybrid beamforming architecture and the recently proposed integrated access and backhaul (IAB) networks become potential candidates for providing cost and hardware-friendly techniques for 6G wireless networks. In addition, in-band-full-duplex (IBFD) has been recently paid much more research attention since it can make the transmission and reception occur in the same time and frequency band, which nearly doubles the communication spectral efficiency (SE) compared with state-of-the-art half-duplex (HD) systems. Since 6G will explore sensing as its new capability, future wireless networks can go far beyond communications. Motivated by this, the development of integrated sensing and communications (ISAC) systems, where radar and communication systems share the same spectrum resources and hardware, has become one of the major goals in 6G. This PhD thesis focuses on the design and analysis of IBFD-IAB wireless networks in the frequency range 2 (FR2) band (≥ 24.250 GHz) at mmWave frequencies for the potential use in 6G. Firstly, we develop a novel design for the single-cell FR2-IBFD-IAB networks with subarray-based hybrid beamforming, which can enhance the SE and coverage while reducing the latency. The radio frequency (RF) beamformers are obtained via RF codebooks given by a modified matrix-wise Linde-Buzo-Gray (LBG) algorithm. The self-interference (SI) is cancelled in three stages, where the first stage of antenna isolation is assumed to be successfully deployed. The second stage consists of the optical domain-based RF cancellation, where cancellers are connected with the RF chain pairs. The third stage is comprised of the digital cancellation via successive interference cancellation followed by minimum mean-squared error (MSE) baseband receiver. Multiuser interference in the access link is cancelled by zero-forcing at the IAB-node transmitter. The proposed codebook algorithm avoids undesirable low-rank behaviour, while the proposed staged-SI cancellation (SIC) shows satisfactory cancellation performance in the wideband IBFD scenario. However, the system performance can be affected by the hardware impairments (HWI) and RF effective channel estimation errors. Secondly, we study an FR2-IBFD-ISAC-IAB network for vehicle-to-everything communications, where the IAB-node acts as a roadside unit performing sensing and communication simultaneously (i.e., at the same time and frequency band). The SI due to the IBFD operation will be cancelled in the propagation, analogue, and digital domains; only the residual SI (RSI) is reserved for performance analysis. Considering the subarray-based hybrid beamforming structure, including HWI and RF effective SI channel estimation error, the unscented Kalman filter is used for tracking multiple vehicles in the studied scenario. The proposed system shows an enhanced SE compared with the HD system, and the tracking MSEs averaged across all vehicles of each state parameter are close to their posterior Cramér-Rao lower bounds. Thirdly, we analyse the performance of the multi-cell wideband single-hop backhaul FR2-IBFD-IAB networks by using stochastic geometry analysis. We model the wired-connected next generation NodeBs (gNBs) as the Matérn hard-core point process (MHCPP) to meet the real-world deployment requirement and reduce the cost caused by wired connection in the network. We first derive association probabilities that reflect how likely the typical user-equipment is served by a gNB or an IAB-node based on the maximum long-term averaged biased-received-desired-signal power criteria. Further, by leveraging the composite Gamma-Lognormal distribution, we derive results for the signal to interference plus noise ratio coverage, capacity with outage, and ergodic capacity of the network. In order to assess the impact of noise, we consider the sidelobe gain on inter-cell interference links and the analogue to digital converter quantization noise. Compared with the HD transmission, the designated system shows an enhanced capacity when the SIC operates successfully. We also study how the power bias and density ratio of the IAB-node to gNB, and the hard-core distance can affect system performance. Overall, this thesis aims to contribute to the research efforts of shaping the 6G wireless networks by designing and analysing the FR2-IBFD-IAB inspired networks in the FR2 band at mmWave frequencies that will be potentially used in 6G for both communication only and ISAC scenarios

    Dual-Function Radar Communications via Frequency-Hopping Code Selection

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    Dual-function radar communications (DFRC) systems serve an indispensable role within emerging paradigm shifts combining sensing modalities with information exchange. Utilising an integrated waveform, the spectral and spatial degrees of freedom (DoF) of the host radar platform are exploited to embed information symbols into the radar waveform. Furthermore, DFRC systems are beginning to embed the information in the fast-time, i.e. within the radar pulse. One method involves the use of orthogonal frequency-hopping (FH) waveforms in conjunction with multiple-input multiple-output (MIMO) radar arrays. While the secondary communications function is achieved, modulating the radar fast-time comes at the expense of the primary sensing operation. In this dissertation, we study the implementation of a novel information embedding scheme for frequency-hopped MIMO (FH-MIMO) DFRC applications. We first develop a generalised framework which unifies existing FH-MIMO DFRC schemes. We then expose new methods of fast-time information embedding, such as the frequency-hopping code selection (FHCS) scheme. We also design hybrid information embedding strategies which enable significantly higher bit rates at no further expense of the radar. Then, we characterise the communications performance of the FHCS scheme exposed by this generalised framework. We identify significant aspects of FHCS signalling which relate to index modulation schemes as a whole, such as the truncation of the symbol dictionary. We formulate an optimisation relating the maximum transform-limit with the achievable communications symbol rate and bit rate. Following this, we address the issue of symbol detection as it pertains to index modulation schemes utilising truncated codebooks. We design a low-complexity communications receiver for the FHCS scheme which ensures valid membership of the estimated symbol to the allowed communications constellation. Furthermore, we derive expressions which show that the probability of symbol error reduces in those cases where truncated dictionaries are employed. Finally, we analyse the performance of the integrated FHCS waveform from the perspective of the primary radar operation. We establish a measure which enables the analysis of the average ambiguity function across all realisations of the permuted symbol dictionary. We also derive the performance of the radar receiver operating characteristics (ROC), including the false-alarm and detection probabilities

    Physical Layer Security in Integrated Sensing and Communication Systems

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    The development of integrated sensing and communication (ISAC) systems has been spurred by the growing congestion of the wireless spectrum. The ISAC system detects targets and communicates with downlink cellular users simultaneously. Uniquely for such scenarios, radar targets are regarded as potential eavesdroppers which might surveil the information sent from the base station (BS) to communication users (CUs) via the radar probing signal. To address this issue, we propose security solutions for ISAC systems to prevent confidential information from being intercepted by radar targets. In this thesis, we firstly present a beamformer design algorithm assisted by artificial noise (AN), which aims to minimize the signal-to-noise ratio (SNR) at the target while ensuring the quality of service (QoS) of legitimate receivers. Furthermore, to reduce the power consumed by AN, we apply the directional modulation (DM) approach to exploit constructive interference (CI). In this case, the optimization problem is designed to maximize the SINR of the target reflected echoes with CI constraints for each CU, while constraining the received symbols at the target in the destructive region. Apart from the separate functionalities of radar and communication systems above, we investigate sensing-aided physical layer security (PLS), where the ISAC BS first emits an omnidirectional waveform to search for and estimate target directions. Then, we formulate a weighted optimization problem to simultaneously maximize the secrecy rate and minimize the Cram\'er-Rao bound (CRB) with the aid of the AN, designing a beampattern with a wide main beam covering all possible angles of targets. The main beam width of the next iteration depends on the optimal CRB. In this way, the sensing and security functionalities provide mutual benefits, resulting in the improvement of mutual performances with every iteration of the optimization, until convergence. Overall, numerical results show the effectiveness of the ISAC security designs through the deployment of AN-aided secrecy rate maximization and CI techniques. The sensing-assisted PLS scheme offers a new approach for obtaining channel information of eavesdroppers, which is treated as a limitation of conventional PLS studies. This design gains mutual benefits in both single and multi-target scenarios

    Complexity Reduction in Beamforming of Uniform Array Antennas for MIMO Radars

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    Covariance matrix design and beamforming in multiple-input multiple-output (MIMO) radar systems have always been a time-consuming task with a substantial number of unknown variables in the optimization problem to be solved. Based on the radar and target conditions, beamforming can be a dynamic process and in real-time scenarios, it is critical to have a fast beamforming. In this paper, we propose a beampattern matching design technique that is much faster compared to the well-known traditional semidefinite quadratic programming (SQP) counterpart. We show how to calculate the covariance matrix of the probing transmitted signal to obtain the MIMO radar desired beampattern, using a facilitator library. While the proposed technique inherently satisfies the required practical constraints in covariance matrix design, it significantly reduces the number of unknown variables used in the minimum square error (MSE) optimization problem, and therefore reduces the computational complexity considerably. Simulation results show the superiority of the proposed technique in terms of complexity and speed, compared with existing methods. This superiority is enhanced by increasing the number of antennas

    Mismatched Processing for Radar Interference Cancellation

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    Matched processing is a fundamental filtering operation within radar signal processing to estimate scattering in the radar scene based on the transmit signal. Although matched processing maximizes the signal-to-noise ratio (SNR), the filtering operation is ineffective when interference is captured in the receive measurement. Adaptive interference mitigation combined with matched processing has proven to mitigate interference and estimate the radar scene. A known caveat of matched processing is the resulting sidelobes that may mask other scatterers. The sidelobes can be efficiently addressed by windowing but this approach also comes with limited suppression capabilities, loss in resolution, and loss in SNR. The recent emergence of mismatch processing has shown to optimally reduce sidelobes while maintaining nominal resolution and signal estimation performance. Throughout this work, re-iterative minimum-mean square error (RMMSE) adaptive and least-squares (LS) optimal mismatch processing are proposed for enhanced signal estimation in unison with adaptive interference mitigation for various radar applications including random pulse repetition interval (PRI) staggering pulse-Doppler radar, airborne ground moving target indication, and radar & communication spectrum sharing. Mismatch processing and adaptive interference cancellation each can be computationally complex for practical implementation. Sub-optimal RMMSE and LS approaches are also introduced to address computational limitations. The efficacy of these algorithms is presented using various high-fidelity Monte Carlo simulations and open-air experimental datasets

    Millimetre-wave radar development for high resolution detection

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    Automotive technology today is focusing on autonomous vehicle development. The sensors for these systems include radars due to their robustness against adverse weather conditions such as rain, fog, ash or snow. In this constant search for advancement, high resolution systems play a central role in target detection and avoidance. In this PhD project, these methods have been researched and engineered to leverage the best radar resolution for collision avoidance systems. The first part of this thesis will focus on the existing systems consisting of the state-of-the-art at the time of writing and explain what makes a high resolution radar and how it can cover the whole field of view. The second part will focus on how a non-uniform sparse radar system was simulated, developed and benchmarked for improved radar performance up to 40% better than conventional designs. The third part will focus on signal processing techniques and how these methods have achieved high resolution and detection: large virtual aperture array using Multiple Input Multiple Output (MIMO) systems, beampattern multiplication to improve side-lobe levels and compressive sensing. Also, the substrate-integrated waveguide (SIW) antennas which have been fabricated provide a bandwidth of 1.5GHz for the transmitter and 2GHz at the receiver. This has resulted in a range resolution of 10 cm. The four part of this thesis presents the measurements which have been carried out at the facilities within Heriot-Watt University and also at Netherlands Organisation for Applied Scientific Research (TNO). The results were better than expected since a two transmitter four receiver system was able to detect targets which have been separated at 2.2â—¦ in angle in the horizontal plane. Also, compressive sensing was used as a high resolution method for obtaining fine target detection and in combination with the multiplication method showed improved detection performance with a 20 dB side-lobe level suppression. The measurement results from the 6-months placements are presented and compared with the state-of the art, revealing that the developed radar is comparable in performance to high-grade automotive radars developed in the industry
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